Availability of iron and major nutrients for phytoplankton in the northeast Atlantic Ocean

Because of recent findings that Fe is a limiting factor for phytoplankton activity even at relatively high dissolved iron (DFe) concentrations, the potential importance of Fe limitation was revisited in the northeast Atlantic Ocean (39–458N, 17–218W). We report data gathered during deck incubation experiments performed at three stations in February–March 2001 with surface seawater containing DFe concentrations of ;0.40 nmol L21. At all stations, Fe addition enhanced phytoplankton growth. Fe limitation was moderate and occurred simultaneously with limitation by major nutrients. This was clearly demonstrated for diatoms that were colimited by orthosilicic acid. Micro-, nano-, and picoplankton benefited from Fe enrichment. Experiments performed with the trihydroxamate siderophore desferrioxamine mesylate B (DFOB) indicated that Fe reserves exist within the cells, especially within the larger cells. This reserve could result from luxurious storage of Fe by colimited cells during episodic atmospheric deposition of Saharan dust. Simulating concentrations of dust resulting from aerosol deposition in well-stratified surface waters, we determined that the solubility of Saharan dust was very low (,0.1% w/w) but the amount of DFe released in seawater was sufficient to relieve the Fe limitation of the ambient phytoplankton community. During the last decade, most biogeochemical studies dedicated to Fe limitation of phytoplankton growth have been carried out in regions characterized by low dissolved iron (DFe) concentrations, such as the equatorial Pacific Ocean (Coale et al. 1996) or the Southern Ocean (de Baar et al. 1995; Boyd et al. 2000; Blain et al. 2001). Recent studies in the Californian coastal upwelling (Hutchins et al. 1998) and the Mediterranean Sea (Sarthou and Jeandel 2001) have shown that even in regions with DFe concentrations one or two orders of magnitude higher than in offshore high-nutrient–low-chlorophyll (HNLC) areas, Fe could be an important factor controlling phytoplankton growth, phytoplankton community structure, and nutrient drawdown. In light of these new findings, the case of the North Atlantic must be revisited.